Multi-cylinder, two-cycle internal combustion engines are commonly used in the marine industry for propulsion, (e.g., in outboard motors and in jet propelled watercraft). In conventional two-cycle internal combustion engines having carburetors, the configuration and position of transfer ports and exhaust ports are selected to optimize engine performance under normal operating conditions at medium or high speeds. In some carbureted engines, the selected configuration and position of transfer ports causes rough idling. In these engines, it is known to notch the exhaust port to provide for good idling stability.
Emissions from conventional two-cycle engines having carburetors contain excessive amounts of unburned hydrocarbons for soon to be implemented environmental regulations, especially when operating at low speeds or idle. In a two-cycle engine having a carburetor, fuel is mixed with intake air at the carburetor upstream of the combustion cylinders. Therefore, air used to scavenge exhaust out of the combustion cylinder is mixed with fuel. Because the scavenging process is not perfect, some of the incoming fuel/air mixture passes directly through the exhaust port without being combusted. This phenomenon is called short-circuiting, and is a significant source of unburned hydrocarbons in emissions from conventional two-cycle engines.
Replacing carburetors with electronic fuel injectors that inject fuel directly into the combustion cylinder substantially reduces the amount of unburned hydrocarbons in the engine exhaust as long as fuel injection is timed and coordinated properly. In crankcase fuel injected engines it is not necessary to notch the exhaust port to provide for good idling stability. In high-pressure direct fuel injection systems, fuel is typically injected after the piston has covered the exhaust port thus assuring that injected fuel will not short-circuit through the exhaust port before the scavenging process is completed. However, as the piston compresses the volume in the piston cavity above the top of the exhaust port, pressure within the combustion chamber increases greatly, and pressure fluctuations within the cylinder can be significant and erratic. Therefore, high-pressure direct fuel injection systems normally implement sophisticated fuel injection techniques (e.g. air-assisted fuel injection requiring the use of an air compressor) to facilitate reliable fuel injection into the high-pressure environment within the piston cavity during compression.
On the other hand, cylinder wall fuel injection (CWI) systems inject fuel into the combustion chamber before the piston covers the exhaust port when the pressure in the combustion chamber is more nearly ambient. Cylinder wall fuel injection systems are more practical because CWI systems can use conventional, low-pressure automotive-type fuel injectors (e.g. 40 to 100 psi), and typically do not need sophisticated pumps or the like to maintain proper fuel injection control. Accurate fuel injection depends in large part on whether the pressure drop across the fuel injector into the piston cavity is predictable. If the pressure drop across the fuel injector is not predictable, it is difficult or even impossible for an electronic controller to accurately meter fuel injection for optimum performance, emissions, etc. by merely opening and closing the injector at the proper time.
Even though pressure fluctuations during fuel injection in low-pressure cylinder wall fuel injection (CWI) systems are not as severe as in high-pressure direct fuel injection systems, abrupt pressure gradients occurring during the piston up-stroke before the exhaust port is fully closed can have adverse effects on fuel injection control in low-pressure cylinder wall fuel injection (CWI) systems. For instance, cylinder pressure may begin to rise before the exhaust port is closed when the engine is at idle or low speeds. Cylinder pressure begins to rise even earlier when the engine is operating at medium or high speeds due to exhaust tuning effects. Therefore, fuel injection into the cylinder late in the cycle using a low-pressure cylinder wall fuel injection system is unreliable.
In two-cycle CWI engines or in carbureted two-cycle engines, the configuration and position of transfer ports and exhaust ports are selected to optimize engine performance under normal operating conditions at medium or high speeds. However, in order to further optimize engine running quality and/or emissions (especially at idle), it may be desirable to inject fuel into the cylinder relatively late in the cycle. This is difficult to do reliably because the abrupt pressure gradients begin in the cylinder at and just after exhaust port closure.